Distribution transformers, which step down a substantially high voltage, in the range of 2400 volts to 21000 volts, to a relatively low voltage, in the range of 120 to 240 volts, are used extensively for distributing electrical power within a service area. These transformers operate by applying the substantially high voltage to a primary winding at a primary side, thereby producing the relatively low voltage on a secondary winding at a secondary side. During operation, however, the distribution transformers are constantly exposed to fault conditions, for example, conditions caused by shorts across distributions lines, internal shorts, or overheating. If not protected, the fault conditions, which are usually manifested by increased heat, may damage or even destroy a distribution transformer.
In order to protect the transformers, fault sensing circuit breakers are widely employed in the power industry. Upon detecting a fault condition, for example, based on a sensed temperature, a circuit breaker isolates the transformer from other power circuitry by breaking a faulty path between two serially connected breaker contacts. Most circuit breakers used in the power industry are secondary circuit breakers, which isolate the secondary side of the transformer. For example, one known conventional secondary circuit breaker incorporates a bi-metal that upon exposure to increased heat bends to break the faulty path. The secondary circuit breakers are, however, inefficient. This inefficiency is largely due to the impedance a secondary circuit breaker presents to the flow of a substantially high current, which is produced at the secondary side by stepping down the substantially high voltage applied to the primary side.
In order to reduce the inefficiency associated with the secondary circuit breakers, primary circuit breakers, which isolate the primary side of a transformer, have been used. Because of a low current flow on the primary side, a primary circuit breaker dissipates much less energy than a secondary circuit breaker. However, unlike the secondary circuit breaker, which breaks a low-voltage path, the primary circuit breaker must break a substantially high-voltage path, i.e., a path with a voltage in the range of 2400 volts to 21,000 volts. When such a high-voltage path is broken, an arc is generated having a length proportional to the voltage level of the broken path.
For safety reasons, the generated arc must be extinguished as rapidly as possible. As a result, conventional primary circuit breakers are equipped with an arc-extinguishing chamber that is immersed in an insulating fluid, also known as transformer oil, which has a dielectric property formulated for extinguishing the arc. While extinguishing the arc, however, the heat produced by the arc breaks the insulating fluid into an expanding gaseous state that must be dissipated to prevent pressure built up in the chamber and a possible circuit breaker explosion.
FIG. 1 shows a cross sectional view of a conventional primary circuit breaker 10, which is disclosed in U.S. Pat. Nos. 4,435,690, 4,611,189, and 4,591,816. The circuit breaker 10 includes an interrupt assembly 12 that is actuated by an external latch mechanism 14 for closing and opening the electrical path between two breaker contacts 16. The circuit breaker 10 is tripped by a temperature sensing device 18, which is responsive to an increase in temperature due to a fault condition. The interrupt assembly 12 includes a central core 20 formed of a molded arc extinguishing material which is enclosed within a glass reinforced plastic sleeve 22. The core 20 includes an elongated bore 24 that is integrated with a circular base 26 at the bottom and a circular cap 28 at the top. The electrical path between the first and second circuit breaker contacts 16 is opened and closed by a conductive rod 30 that under the control of the latch mechanism 14, moves reciprocally within the elongated bore 24.
Under the arrangement of FIG. 1, the space between the base 26 and the cap 28 defines a single arc-chamber 32, which is open to the elongated bore 24 through a number of openings 34. The openings 34, which are disposed along the length of the bore 24, allow the insulating fluid to dielectrically insulate the path of the conductive rod 30 as it travels downward along the core 20. As a result, the generated gases can expand into the arc-chamber 32 and remain confined within the surrounding walls provided by the sleeve 22. A relief port 36 is provided on the cap 28 for the discharge of oil and/or gases from the arc-chamber 32. The port 36 also operates as an entry port for the insulating fluid, allowing it to enter into the arc-chamber 32, when the circuit breaker 10 is immersed into the insulating fluid.
However, it is desirable to reduce the size and manufacturing cost of the conventional primary circuit breaker of FIG. 1. By reducing the size of the circuit breaker, a shorter fluid tank with less fluid may be used for immersing the circuit breaker. Also, a smaller fluid tank would significantly facilitate the handling of the circuit breaker, for example, during installation and maintenance. In addition, it is also desirable to reduce the manufacturing cost of the circuit breaker by reducing the number of parts used for assembling the conventional circuit breaker. Moreover, afer frequent exposure to arcs, a carbon layer is formed along the length of the elongated bore 24. Due to the conductive property of the carbon layer, the insulating property of the bore 24 may diminish, resulting in early failure of the circuit breaker 10.
Therefore, there exists a need for a small and durable primary circuit breaker that can be manufactured cost effectively.